This invention relates to an improved method of preparing uniform, ultra-thin coatings on substrates by sol-gel processing and to coated articles prepared thereby.
There is presently a growing need for protecting various substrates against corrosion and other environmental effects by the use of thin films resistant to these effects. The most common methods of applying such protective films to substrates has been by chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), sputtering and thermal oxidation. All have disadvantages, especially with regard to the low temperature preparation of dielectric films.
Another method for applying thin protective coatings to a variety of substrates is known as sol-gel processing. Sol-gel processing is a method whereby small molecules can be converted into polymeric or ceramic materials. Depending on the nature of the monomers one can form anything between organic polymers such as polydimethylsiloxane and inorganic ceramics such as silicon dioxide. Typically one mixes metal alkoxides of network forming cations, e.g., Si, Al, B, Ti, P, in an appropriate solvent such as an alcohol with water and a catalyst. In the catalyzed solution the alkoxides are partially or completely hydrolyzed and then polymerized to form molecules of a glass-like oxide or ceramic network linked by bridging oxygen atoms. The overall process that takes place using tetraethoxysilicon as the metal alkoxide involves two main reactions. The first is the hydrolysis of the alkoxide to form the hydroxy-containing species (e.g. 1). EQU Si--(OCH.sub.2 CH.sub.3).sub.4 +H.sub.2 O.fwdarw.HO--Si--(OCH.sub.2 CH.sub.3).sub.3 +CH.sub.3 CH.sub.2 OH
This reaction can be repeated, depending upon the conditions, until all of the alkoxide groups have been replaced.
The second reaction is the condensation of the hydroxy-containing species to form metal-oxygen-metal bonds (eq 2). EQU 2 HO--Si--(OCH.sub.2 CH.sub.3).sub.3 .fwdarw.(CH.sub.3 CH.sub.2 O).sub.3 --Si--O--Si--(OCH.sub.2 CH.sub.3).sub.3 +H.sub.2 O
This reaction can proceed until all of the hydroxide groups have been used up, resulting in a network of ceramic-type bonds (eq 3). EQU n(CH.sub.3 CH.sub.2 O).sub.3 --Si--O--Si--(OCH.sub.2 CH.sub.3).sub.3 .fwdarw.2n[SiO.sub.2 ]
The chemistry of sol-gel processing is well documented in the prior art. See, e.g., (1) Brinker et al, "Sol-gel Transition in Simple Silicates", J. Non-Cryst. Solids, 48 (1982) 47-64; (2) Brinker et al, "Sol-gel Transition in Simple Silicates II", J. Non-Cryst. Solids, 63 (1984) 45-59; (3) Schaefer et al, "Characterization of Polymers and Gels by Intermediate Angle X-ray Scattering", presented at the International Union of Pure and Applied Chemists MACRO'82, Amherst, Mass., Jul. 12, 1982; (4) Pettit et al, Sol-Gel Protective Coatings for Black Chrome Solar Selective Films, SPIE Vol. 324, Optical Coatings for Energy Efficiency and Solar Applications (pub. by the Society of Photo-Optical Instrumentation Engineers, Bellingham, Wash.) (1982) 176-183; (5) Brinker et al, "Relationships Between the Sol to Gel and Gel to Glass Conversions", Proceedings of the International Conference on Ultrastructure Processing of Ceramics, Glasses, and Composites, (John Wiley and Sons, N.Y.) (1984); (6) Brinker et al, "Conversion of Monolithic Gels to Glasses in a Multicomponent Silicate Glass Sytem", J. Materials Sci., 16 (1981) 1980-1988; (7) Brinker et al, "A Comparison Between the Densification Kinetics of Colloidal and Polymeric Silica Gels", Mat. Res. Soc. Symp. Proc. Vol. 32 (1984), 25-32; all of which disclosures are incorporated by reference herein.
One of the drawbacks of these prior art sol-gel processes is that they are not able to provide ultra-thin films of acceptable completeness and uniformity, particularly at average film thicknesses below 500 .ANG.. Consequently, the environmental and corrosion protection provided substrates by the prior art sol-gel coating processes is less than satisfactory.
It is an object of the present invention to coat substrates with an ultra-thin film which is complete and uniform even at thicknesses ranging from 5 to 500 .ANG..
Yet another object of the invention is to provide synthetic polymer substrates protected against adverse environmental effects by an ultra-thin film.
Another object of the invention is to provide corrosion-resistant surfaces coated with an ultra-thin film that is invisible to the naked eye.